A welding sleeve is disclosed in Swiss application No. 396 536 in which a coil is fixedly disposed within a tubular body by placing the coil within an injection mold for forming the body or extruding the body over the coil. Such welding sleeve is produced in a complicated manner and has a good safety factor as a result of the resistance wire being encased and partially coated to prevent short circuits and accidents during handling of the welding sleeve when charged with voltage without being plugged in conductive tube elements. However, the quality of the welding connection for commercially available pipes is insufficient.
The commercially available pipes are somewhat oval in cross section and have wide measurement tolerances for the outside diameter. In order to be able to slide such pipes without problems into the sleeve, the inside diameter of the coil and of the sleeve must be made correspondingly large. The contraction capacity of a tubular body which has been injection molded or extruded onto the coil is about 1.5%. In practice, this low contraction capacity and the required play between sleeve and pipe, particularly in case of a combination of extreme tolerances, develops insufficient welding pressure at the periphery resulting in a poor welding connection.
In order to be able to balance out such tolerances and to produce a sufficient welding pressure, it has been known to widen the tubular bodies by heat stretching. In that case radial contraction strains are frozen in the tubular body, which become free upon heating of the welding sleeve by means of the resistance wire, causing a shrinking of the welding sleeve.
Swiss Pat. No. 544 906 discloses a welding sleeve with an extruded annular body widened in such a manner. This welding sleeve is provided with a bare resistance wire which is inserted into a groove produced after widening of the body inside diameter by incision. However the built-in contraction strains are lost as a result of subsequent processing or they no longer become free in a controlled manner. This sleeve is also disadvantageous, since the bare resistance wire is unsafe and since the production of the welding sleeve by mechanical processing and the insertion of the resistance wire into the groove is very complicated and expensive.
Furthermore, German application No. 2,302,458 discloses a welding sleeve in which a bare heating wire is applied to core jacket segments disposed on a mandrel with the required spacing of windings. A widening device is inserted through the windings of the wire and into a pipe section heated on the outside wall. Then the inside wall is heated to about 180.degree. C. by heating the mandrel to connect the bare heating wire with the pipe section. However, by this heating the contraction strains in the pipe section are liberated for the greatest part and are thus not available for the bridging of larger pipe tolerances during the production of the welding connection.
In the welding sleeve shown in application WO-A No. 79/01018, the resistance heating wire is covered up by a part of the material on the inside diameter of the piece of pipe. The production of electric heat in the resistance wire causes flowing of the material in its vicinity and penetration of the windings into the material. In this case too, the contraction strains existing in the piece of pipe, are freed, which leads to the described disadvantages.